Use absolute path to /etc/hosts

Not providing a path to hosts as an argument or not running from the root directory causes a file not found error.
Don't execute when serf invokes member-failed
because 'member-failed' doesn't mean that a host disappears.
Change permission of $tmp_file into 0644

File::Temp::tempfile create a temperary file which has 0600.
/etc/hosts is 0644 by default.
fix Two-argument "open" to tree
care regexp meta characters, especially '.'
Merge pull request #3 from tacahilo/remove-case-of-member-failed

Don't execute when serf invokes member-failed
Merge pull request #4 from tacahilo/change-permission-of-tmp-file-into-0644

Change permission of $tmp_file into 0644
Merge pull request #6 from Songmu/fix/quotemeta

care regexp meta characters, especially '.'
Merge branch 'master' into fix/open-style

Conflicts:
	event_handler.pl
Merge branch 'patch-1' of github.com:rvangundy/serf-hosts into rvangundy-patch-1

Conflicts:
	event_handler.pl
Merge branch 'rvangundy-patch-1'
Merge pull request #5 from Songmu/fix/open-style

fix Two-argument "open" to tree